Measurements of a device under test (DUT) using a VNA may not always be performed in a desired test environment. This is because it may be too time intensive and/or costly to measure a DUT in a desired test environment. Accordingly, a DUT is often measured in a different environment for reasons of expediency and/or practicality, thereby requiring the use of embedding or de-embedding techniques to correct the effects of the test environment. For example, a DUT may be in a test fixture when measurements of the DUT are made, thereby requiring the removal of the effects of the fixture from the measured data for a truer picture of actual DUT performance. De-embedding techniques allows this task (i.e., removal of effects) to be performed computationally. In another example, a customer may desire to see what the performance of a DUT would be with a specific matching network attached. However it may be impractical to attach the matching network during manufacturing for cost reasons. Embedding techniques allow this task (i.e., attaching the matching network) to be performed computationally.
In an ideal de-embedding problem, a test fixture to be de-embedded can be characterized by placing multiple calibration standards at input and output planes of the fixture. In practice, this is often difficult due to the nature of the media at one or more interfaces (e.g., a launch onto a PC board). This problem is compounded in multiport problems in that there may be intra-port coupling that needs to be taken into account.
As just explained, a structure to be de-embedded can be a test fixture surrounding a multiport DUT. The classical approach has been to treat all ports of the fixture as uncoupled, in which case standard two port de-embedding techniques may be used, as explained with reference to FIG. 1. In the classic approach, the path from each fixture port to the DUT is treated as an independent two port network, also referred to as a fixture part. This can be appreciated from FIG. 1, which schematically shows a 4-port DUT 102 connected to four fixture parts 1041, 1042, 1043 and 1044. Typically, three or more reflect standards (also known as calibration standards) would be connected at the DUT plane 107 for each fixture part 104n, From this, the S-parameters of each fixture part can be deduced. This could require a fair amount of standards development, which could be difficult depending on the media near the DUT. Also, this may require a reciprocity assumption about the fixture, as explained in an article by R. Bauer and P. Penfield, entitled “De-embedding and Unterminating,” IEEE Trans. On MTT, vol. MTT-22, March 1974, pp. 282-288, which is incorporated herein by reference.
Still using the two port construct, one could try to just use a thru pair standard, which connect fixture part 1041 to fixture part 1043, and connects fixture part 1042 to fixture part 1044. Additional assumptions must be made in this case, including: most mismatch concentrated near the external launch point and reasonably well-matched overall; and well-matched at the DUT interface planes.
However, if the ports of the fixture have any coupling (i.e., if fixture parts are coupled), this procedure breaks down since the de-embedding must be treated as a 4 (or more) port network, instead of a 2 port network. Thus, even if the assumptions stated above can be met, there will be accuracy issues due to the mishandling of coupling.
A complete solution to the problem would be a variant on extraction, which is explained, e.g., for example, in Anritsu Application Note 11410-000278, entitled “Embedding/De-embedding,” May 2002, pp. 12-13. In this technique, a full N-port calibration is accomplished at the inner planes of the fixture as well as the outer planes. This can be explained with reference to FIG. 2.
Referring to FIG. 2, a DUT 202 is shown surrounded by a fixture 200. Here, two full calibrations can be performed, one using a outer calibration plane 206, and one using an inner calibration plane 207, which allows S-parameters of the fixture to be extracted. Using this technique some port coupling within the fixture 200 is allowed, but caution is required.
This full extraction process, can however, be troublesome in that a full set of calibration standards must be developed at the inner plane 207. This can be expensive and may have accuracy problems since the media at the inner planes may not be well characterized. The number of standards used may be dependent on the level of intra-fixture coupling that one is trying to extract and convergence issues can arise at very high coupling levels.
Accordingly, there is still a need for an extraction procedure that can handle some level of port coupling within the fixture, require a minimum of standards development at the inner plane, and still remove the fixture effects with reasonable accuracy.